As is well known in decolorizing rubbery hydrogenated polymers, it is a conventional expedient to use a mineral acid or a mono- or polycarboxylic acid to remove the deep color imparted by hydrogenation catalysts. This treatment results in transparent but yellowish polymers. With regard to resinous types of copolymers, particularly those described in U.S. Pat. Nos. 3,639,517 (Kitchen et al.) and 4,091,053 (Kitchen), the color is mainly due to the presence of initiation agents as well as to the presence of agents employed in the process which is used to couple the polymer chains (vinyl aromatic-conjugated diene) to each other. It is very important, not only for appearance but also for uses of the polymers with foodstuffs, to make this yellowish coloration disappear and to obtain perfectly transparent and colorless copolymers.
Methods are known for obtaining transparent copolymers by treating solutions of coupled copolymers with CO.sub.2 and water. However, this necessitates distillation of the solvent, to separate water and CO.sub.2 from the solvent, before it can be recycled for the the initiator. A consideration is not only transparency and decoloration, but also the fact that the polymer should retain its impact strength properties. It is also known that it is necessary to add to these compositions an anti-oxidant additive to stabilize the butadiene phase during the polymer transformation stage. Indeed the temperatures employed during this stage may involve degradation of the polybutadiene phase; however, the type of additive used may cause discoloration of the polymeric material.
It has also been proposed in European Patent Application Publication No. 84,795 (Mocygemba) to treat the copolymers with dicarboxylic acids. It is understood that with this kind of process it is possible to satisfy practically all the conditions of decoloration mentioned above, but without providing a solution to the problem resulting from addition of an anti-oxidant. It is therefore desirable to have available a process for treating the vinyl aromatic-conjugated diene copolymers so as to ensure their perfect transparency, to retain appropriate impact strength properties, and moreover to satisfy the extremely demanding conditions involved in food supply, and to give them an anti-oxidant activity without requiring an excessive amount of anti-oxidant additive.